As is well known, a power shovel has a bucket, an arm and a boom as working machines, which are driven by a bucket cylinder, an arm cylinder and a boom cylinder, respectively. In order to move the bucket in desired locus and posture, it is indispensable to simultaneously control expansion and contraction of the respective cylinders.
Accordingly, in order to move the bucket in desired locus and posture, the operator must simultaneously or alternately operate respective operation levers corresponding to the bucket, arm and boom. Hence, skill is needed for their operation.
An inexperienced operator causes increase in unnecessary resistance against excavation by, for example, not directing the front edge of the bucket in the direction of movement, or by making the base plate of the bucket interfere with an excavated surface after excavation.
On the other hand, there have been proposed various kinds of apparatuses for controlling power shovels in which a moving locus (for example, a straight line, a circular are or the like) of the front edge of the bucket and the posture of the bucket for the locus have previously been set, and the bucket, arm and boom are automatically controlled so that the front edge of the bucket moves along the locus.
However, these conventional automatic excavating apparatuses are in general for finishing operation. Very few apparatuses aim at excavating and loading operations. Furthermore, apparatuses for excavating and loading operations are still incomplete from the viewpoint of operation efficiency, operation capability, time required for excavation, and the like. Hence, the relating technique is still immature for being used in an actual apparatus.
Furthermore, in conventional apparatuses, the speed of working machines at the moment of an automatic mode is fixed. No apparatuses have existed in which the speeds of working machines can be arbitrarily changed by a simple operation.
Moreover, in conventional apparatuses, the locus of excavation is fixed. Hence, there is a problem in that, even when a bucket hits hard earth and sand, an obstacle and the like in the course of excavation, the bucket intends to move along an excavation locus which has previously been set, and as a result, relief loss occurs, and efficiency is therefore reduced.
In addition, conventional apparatuses are more or less unsatisfactory from the viewpoint of efficient utilization of pump output. That is, in conventional apparatuses, commands for flow rates for respective working machines are obtained by obtaining the distribution ratio of the flow rate of a pump for respective working machines according to angles of rotation needed for respective working machines, and by distributing the flow rate of the pump determined from actual pump pressure in the distribution ratio. In general, oil supplied from a pump tends to flow toward a working machine having small load. In conventional apparatuses, the values of commands for flow rates calculated from the above-described distribution ratio are input to respective working machines without modification. Hence, oil flows to a working machine having small load in the amount which is more than the amount corresponding to the command for the flow rate, and oil flows to a working machine having large load in the amount which is less than the amount corresponding to the command for the flow rate. As a result, oil is not exactly distributed in accordance with the distribution ratio. Actual flow rates of oil for respective working machines are determined according to relative movement between a pump and valves for working machines, and oil does not flow exactly in the amount corresponding to the values of commands for respective working machines. Hence, the actual values of flow rates become smaller than the sum of the values of commands for flow rates for respective working machines. As a result, relief loss and loss in pump energy are produced, and time for excavation therefore increases.
The present invention has been made in consideration of such circumstances.
It is an object of the present invention to provide a method and an apparatus for controlling the hydraulic cylinders of a power shovel in which the working machines are automatically controlled so as to perform the most suitable operation for excavation by a simple operation, and efficiency for excavating and loading operation can be improved.
It is a further object of the present invention to provide a method and an apparatus for controlling hydraulic cylinders of a power shovel in which automatic excavation can be performed in the most suitable posture and locus of a working machine by a simple operation of an operation pedal, operation efficiency is therefore improved, and the speeds of working machines can be arbitrarily changed in accordance with the tread angle of the operation pedal.
It is a still further object of the present invention to reduce relief loss by correcting a locus which has been set in accordance with actual load, and to enable to perform excavation of an always constant amount of earth even when the locus has been corrected.
It is still another object of the present invention to provide an apparatus for controlling working machines of a power shovel in which excavation efficiency is improved by driving the working machines effectively utilizing pump output.